VLF transmissions cover the 3-30 kHz range, and thus bandwidth is highly limited. VLF hardware is primarily used to communicate with submarines, often to remind them that, yes, everything is still fine and there’s no need to launch the nukes yet. It’s also used for navigation and broadcasting time signals.

It seems that this human transmission has created a barrier of sorts in the atmosphere that protects it against radiation from space. Interestingly, the outward edge of this “VLF Bubble” seems to correspond very closely with the innermost edge of the Van Allen belts caused by Earth’s magnetic field. What’s more, the inner limit of the Van Allan belts now appears to be much farther away from the Earth’s surface than it was in the 1960s, which suggests that man-made VLF transmissions could be responsible for pushing the boundary outwards.